Field of the Invention
The present invention relates to an image sensor, a control method therefor, and an image capturing apparatus, and more particularly to an image sensor that reads out the signals of a plurality of photodiodes (PD) via a shared readout circuit, a drive method therefor, and an image capturing apparatus.
Description of the Related Art
In the field of image sensors, various technologies have been proposed to improve the light receiving efficiency of photoelectric conversion elements such as photodiodes (PDs) while reducing the size of individual pixels, in response to calls for higher resolution and higher sensitivity. With CMOS image sensors, instead of a configuration in which one readout circuit is provided for every one pixel, a configuration is now often employed in which one readout circuit is shared by a plurality of pixels and that one readout circuit is used sequentially (see Japanese Patent Laid-Open No. 2000-078474). A readout circuit as referred to here includes, for example, constituent elements such as a floating diffusion (FD) portion that temporarily accumulates electric charges transferred from each pixel and an amplification transistor whose gate receives input of the potential of the FD portion and that operates as a source follower circuit with a predetermined load current. By sharing one readout circuit between N pixels, the area required for such constituent elements can be reduced to 1/N and the saved area can be assigned to the PDs, thus enabling light receiving efficiency to be improved.
In recent years, marked progress has also been made toward higher functionality, and image sensors have also been proposed in which a plurality of PDs are provided in one pixel, in order to acquire information that is used in autofocus (AF) employing an on-imaging plane phase detection method. By constituting one pixel corresponding to one microlens with a PD divided horizontally into two, light that has passed through different pupil regions of the imaging optical system can be received by each PD. A shared readout circuit configuration is favorable, even when thus reading out the signals of twice as many PDs as there are pixels of a captured image.
Japanese Patent Laid-Open No. 2014-030148 discloses an image sensor that sequentially reads out electric charges accumulated in two photodiodes (PD) 1 and 2 that are used for phase difference detection to a shared readout circuit as an optical signal S1 and an optical signal S2. Here, the optical signal S1 corresponds to the signal of the PD 1 and the optical signal S2 corresponds to the sum of the signals of the PD 1 and the PD 2. That is, the optical signal S2 is an image signal for image capture that corresponds to light that has passed through all of the pupil regions of the imaging optical system. Therefore, it is possible to generate a captured image easily as required using the optical signal S2, without additional circuitry, while maintaining a pixel configuration that enables phase difference detection. Also, although an increase in random noise is conceivable in the case of generating a captured image by independently reading out the signal charges of the PD 1 and the PD 2 and combining these signal charges, such concerns do not arise.
However, the optical signal S2 is read out following a reset level readout period and the readout period of the optical signal S1, after resetting the FD portion. Therefore, there is a large time difference between the readout period of the optical signal S2 and the reset level readout period in which a signal to be used in correlated double sampling (CDS) is obtained, resulting in a decrease in temporal correlation. Also, since the FD portion does not have a deep buried structure unlike the PDs, a large amount of dark charge is generated in proportion to the time period that has elapsed from reset release. White dots appear on the captured image when there is variation in the generated amount of dark charge between FD portions.